The present invention relates to a method of positioning a control surface to correct for hysteresis, and a control system embodying the method.
Actuators can be used to move a control surface to a required position. The control surface could be a part of the actuator, or could be part of a separate component moved by the actuator. However, due to mechanical effects such as elastic deformation and backlash of the system, it can be difficult to determine the actual position of the control surface following movement, and it is therefore difficult to use an actuator to accurately move the control surface to the required position. One solution to this problem is to directly measure the position of the control surface using a position sensor. However, in some applications such as for example inlet guide vanes in gas turbine engines, there may be insufficient space to install such a sensor. It is therefore necessary to move the actuator a predetermined amount which corresponds to an amount that is predicted to be necessary to move the control surface to the required position.
The difference between a required position and an actual position of a control surface following movement of the actuator by the predetermined amount can depend on previous movements of the actuator. This can be a result of elastic deformation of the actuator and/or control surface and can also be a result of “backlash”, i.e. play in the actuator. As a result, the actual position following movement by the predetermined amount will generally lag behind the required position, especially where the direction of movement is reversed. This effect is known in the art as “hysteresis”.
As shown in FIG. 2, the magnitude of the difference between the required position and the actual position following movement by the predetermined amount varies as the actuator is moved. As shown in FIG. 2, the actual position lags behind the required position when the control surface is moved in both the opening and closing directions.
The present invention provides an improved system that addresses some or all of the aforementioned problems.